To enhance warming of various vehicle system components, thermal energy storage devices have been developed to store thermal energy produced by the vehicle system for later use. These thermal storage devices typically include a phase change material (PCM) that may store a significant amount of thermal energy as latent heat at the phase change temperature of the PCM. In one example approach disclosed in US 2004/0154784, phase change materials such as paraffin wax may be included in the interior of a vehicle to conserve energy while providing heat to the passenger compartment.
However, the inventors herein have recognized potential issues with such systems. Specifically, estimates of the state of charge of such thermal storage devices may be significantly reduced at the phase change temperature of the PCM in systems including only one type of PCM. Because of the latent heat stored in the PCM at its phase change temperature, it may be difficult to estimate the state of charge of the PCM when the PCM is transitioning between phases at its phase change temperature. Further, even in thermal storage devices including more than one PCM, such as the device disclosed in US 2014/0079978, the accuracy of estimates of the state of charge may be reduced when the temperature of coolant exiting the thermal storage device is different than the temperature of the thermal storage device.
For example, the temperature of the coolant exiting the thermal storage device may be different than the temperature of the thermal storage device when the coolant is not warmed to the temperature of the thermal storage device. This may occur when the coolant entering the thermal storage device is significantly colder than the thermal storage device, such that the thermal storage device cannot warm the coolant fast enough to bring it to thermal equilibrium with the thermal storage device before the coolant exits the thermal storage device. Thus, the coolant may not remain in the thermal storage device for long enough to reach thermal equilibrium with the thermal storage device. As such, the temperature of the coolant exiting the thermal storage device may not reflect the actual temperature of the thermal storage device. Therefore, when estimating the state of charge of the battery based on the temperature of coolant exiting the battery, the accuracy of such estimates may be reduced.
As one example, the issues described above may be addressed by a method comprising estimating a temperature of a thermal battery after the battery and coolant included therein have reached thermal equilibrium, and determining a state of charge of the battery based on the estimated temperature and one or more chemical properties of phase change material included in the thermal battery. The temperature of the thermal battery may be estimated based on outputs from a temperature sensor coupled to a coolant outlet of the battery, where the sensor may be configured to measure a temperature of coolant exiting the battery.
In another example, a method for an engine cooling system may comprise stopping coolant flow through a thermal storage device comprising two phase change materials with different melting points for a duration, resuming coolant flow through the thermal storage device after the duration and estimating a temperature of coolant exiting the thermal storage device based on outputs from a temperature sensor positioned proximate a coolant outlet of the device, and calculating a state of charge of the device based on the estimated coolant temperature and one or more chemical properties of the phase change materials. Additionally, the duration may comprise an amount of time for coolant included within the device and internal components of the device including the phase change materials, to reach thermal equilibrium, and where the duration may be calculated based on a most recent coolant temperature measurement and a most recent state of charge estimate of the battery.
In yet another example, a thermal battery system may comprise a thermal storage device including a first phase change material having a first phase change temperature and a second phase change material having a second, different phase change temperature. The thermal battery system may further comprise a coolant valve adjustable between a first position and a second position to selectively couple the thermal storage device to an engine coolant circuit and regulate an amount of coolant circulating through the thermal storage device. Additionally, the thermal battery system may comprise a temperature sensor for estimating a temperature of the device, and a controller with non-transitory computer readable instructions for: estimating a temperature of the device when coolant within the device has stopped for more than a threshold duration, and determining a state of charge of the battery system based on the estimated temperature and one or more chemical properties of the phase change materials. In some examples, the first and second phase change materials may be combined together in a mixture. However, in other examples, the first and second phase change materials may be separated from one another into distinct battery cells.
In this way, the accuracy of estimates of the state of charge of a thermal battery may be increased by temporarily stopping coolant flow through the thermal battery until the battery, its internal components, and coolant included therein, have reached thermal equilibrium. By resuming coolant flow after the coolant and battery have reached thermal equilibrium, and measuring coolant temperature of coolant exiting the battery that is at the temperature of the thermally equilibrated battery, a more direct and accurate measurement of the battery temperature and therefore state of charge may be obtained.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.